Isaiah Shavitt

TL;DR: In this article, the authors reported the development of several new computational approaches within the framework of multi-reference molecular electronic structure methodology and their implementation in the COLUMBUS program system, including the calculation of the analytical MR-CI gradient for excited states based on state-averaged MCSCF orbitals, the extension of the MR-ACPF/AQCC methods to excited states in the context of linear response theory, spin-orbit CI for molecules containing heavy atoms and a massively parallel code for the computation of the one-and two-particle density metrics.

TL;DR: A new method for computing analytic energy gradients for MRSDCI wave functions is discussed, which avoids the “coupled perturbed MCSCF” solutions for each coordinate direction, and avoids the transformation of any derivative-integral quantities from the AO to the MO basis.

TL;DR: The correlation energy as mentioned in this paper is defined as the difference between the Hartree-Fock (HF) limit energy and the exact solution of the nonrelativistic Schrodinger equation.

TL;DR: In this article, three forms of quasidegenerate perturbation theory are discussed and compared in terms of a common general formulation based on a similarity transformation which decouples the model space and complementary space components of the Hamiltonian.

TL;DR: In this paper, coupled cluster doubles and manybody perturbation theory (MBPT) and coupled cluster methods are employed in an investigation of the potential energy surface of H2O in the vicinity of its equilibrium geometry.